1. Field of the Invention
The present invention relates an acceleration sensor including a piezoelectric element.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 6-273439 (corresponding to counterpart U.S. Pat. Nos. 5,515,725 and 5,490,422) discloses an acceleration sensor including a piezoelectric ceramic. Each of two piezoelectric ceramic layers, bonded together, includes three longitudinally aligned regions at two borders at which stress is inverted in the longitudinal direction of the layers when acceleration is applied thereto. These three regions are connected in parallel, and then the two layers are connected in series. FIG. 8A illustrates a charge that is generated when an acceleration G acts on the acceleration sensor, and FIG. 8B is a circuit diagram of the acceleration sensor.
Japanese Unexamined Patent Application Publication No. 2000-121661 (corresponding to counterpart U.S. Pat. No. 6,360,603) discloses another acceleration sensor. Each of two piezoelectric ceramic layers, bonded together, includes three longitudinally aligned regions separated at two borders at which stress is inverted in the longitudinal direction of the layers when acceleration is applied thereto. These three regions are connected in parallel, and then the two layers are connected in parallel. FIG. 9A illustrates a charge that is generated when an acceleration G acts on the acceleration sensor, and FIG. 9B is a circuit diagram of the acceleration sensor.
In the acceleration sensor shown in FIGS. 8A and 8B, cells (1) through (3) in one layer are connected in parallel and cells (4) through (6) in the other layer are connected in parallel. In the acceleration sensor shown in FIGS. 9A and 9B, cells (1) through (6) in the two layers are connected in parallel. In these arrangements, voltages generated in the cells are not summed. Sensitivity to the voltage generated in each layer is low, and the performance of these acceleration sensors is not high enough in applications where high sensitivity is required. Since all six cells (1) through (6) are connected in parallel in the acceleration sensor shown in FIGS. 9A and 9B, the voltage sensitivity thereof is half the voltage sensitivity provided by the acceleration sensor shown in FIGS. 8A and 8B.
To increase the voltage sensitivity, the gain of a voltage amplifier connected to the sensor must be increased. If the gain of the amplifier is increased, a noise component applied to the amplifier is also amplified. Also, the S/N ratio is degraded. Increasing the gain of the voltage amplifier does not serve the purpose of measuring a small acceleration.
In an acceleration sensor including a piezoelectric ceramic, no noise is generated from the piezoelectric ceramic itself. The sources of noise include external noise ingressing between the sensor and the input terminal of a circuit thereof, and internal noise responsive to input conversion voltage noise generated at an input stage of an operational amplifier defining the amplifier.
These noises will now be discussed with reference to a charge amplifier shown in FIG. 10 and a voltage amplifier shown in FIG. 11.
The sensor S here has voltage sensitivity Vs, charge sensitivity Qs, and capacitance Cs. It is well known that the relationship of Qs=Vsxc3x97Cs holds. Overall gains G of the two amplifiers are listed in Table 1. In the charge amplifier, the gain G is proportional to the charge sensitivity Qs. In the voltage amplifier, the gain is proportional to the voltage sensitivity Vs.
The external noise Vn is now considered. The external noise Vn is electrostatically coupled through capacitance Cc to a line running between a sensor S and an input terminal of an amplifier OP, and the magnitude of noise is thus represented by Vn. The external noise Vn appears as Von after being amplified through the amplifier. Since the polarity of noise is not important, all noises are expressed in absolute values in Table 1.
In the charge amplifier, a coupling capacitance Cc and a feedback capacitance C1 form an inverting amplifier. The external noise voltage has a magnitude that is obtained by multiplying Vn by gain (Cc/C1). In the voltage amplifier, noise is voltage divided by a coupling capacitance Cc and capacitance Cs of a sensor, and is then output through a voltage follower.
If G converted noise that represents the magnitude of noise with reference to the level G is defined by the reciprocal of the S/N ratio, the G converted noise becomes a noise voltage/G gain. For example, if the G converted noise is 100 mG, noise as large as 100 mG is continuously generated even if no acceleration appears in the output of the circuit. An acceleration below that level cannot be measured. The G converted noise in each circuit is listed in Table 1. The G converted external noise is not related to the circuit type. The larger the charge sensitivity Qs, the smaller the G converted external noise, and the smaller acceleration is measured.
Table 1 lists internal noise voltage. Internal noise En is generated at a positive input terminal of an operational amplifier OP. A non-inverting amplifier is defined by a capacitance Cs of the sensor S and the feedback capacitance C1, and the output thereof is obtained by multiplying En by the resulting gain. Since the voltage amplifier is a voltage follower of gain one, En is directly output. The G converted internal noise is obtained by dividing noise voltage Von by G gain Vos as listed in Table 1. The G converted internal noise is not related to the circuit type. The larger the voltage sensitivity Vs, the smaller the G converted internal noise, and the smaller acceleration that is measured.
A sensor having a large charge sensitivity and a large voltage sensitivity is a good sensor. The product Qsxc2x7Vs/2 of the charge sensitivity and voltage sensitivity corresponds to energy Es generated by acceleration. To increase the energy Es, the sensor must be large in size. The external noise here depends on the layout on a printed circuit board, and the internal noise depends on an amplifier characteristic of an operational amplifier in use. Depending on the status of a host apparatus of the sensor, more weight is attached to one of the charge sensitivity Qs and the voltage sensitivity Vs than the other while the energy Es, which is the product of the charge sensitivity and the voltage sensitivity, is maintained. The above-disclosed sensors with the large charge sensitivity thereof are appropriate for use in applications where external noise level is high, but are not appropriate for use in applications where internal noise level is high because of the small voltage sensitivity thereof.
In order to overcome the problems described above, preferred embodiments of the present invention provide an acceleration sensor which increases a voltage sensitivity without reducing energy, which is equal to the product of charge sensitivity and voltage sensitivity, as much as possible, and which is appropriate in an operating environment where internal noise is high in level.
An acceleration sensor of a preferred embodiment of the present invention includes a piezoelectric element and a support member for supporting the piezoelectric element at both longitudinal ends thereof. The piezoelectric element includes a laminate of at least two piezoelectric layers. Each of the least two external piezoelectric layers of the piezoelectric element includes three longitudinally aligned regions separated at two borders where stress is inverted in the longitudinal direction of the piezoelectric element when acceleration is applied. Each of a plurality of cells defined by a respective region is polarized in the same direction of thickness in each of the two external piezoelectric layers. Electrodes are arranged on the top and bottom major surfaces and between the layers of the piezoelectric element so that the three longitudinally aligned cells are electrically connected in series.
The acceleration sensor of preferred embodiments of the present invention including a serial electrical connection of the three cells in the at least two external piezoelectric layers in the longitudinal direction presents a voltage sensitivity that is relatively higher than a conventional sensor in which the cells are connected in parallel. Although a charge sensitivity decreases, the energy equal to the product of the charge sensitivity and the voltage sensitivity suffers from no significant change as in the conventional sensor. The voltage sensitivity is thus increased while a decrease in the generated energy is controlled. Thus, preferred embodiments of the present invention provide an acceleration sensor that is appropriate for use in applications where internal noise is high.
The three cells in the at least two external piezoelectric layers or six cells in the at least two piezoelectric layers are serially connected. If an insulation resistance between electrodes in any particular cell is degraded, the remaining cells maintain the performance thereof. The entire sensor is thus less sensitive to individual cell failure. The thickness of the piezoelectric layer is reduced and sensitivity is increased as the degree of required insulation is accordingly alleviated.
In the conventional acceleration sensor, three separate cells arranged in the longitudinal direction are inverted in polarization between two adjacent cells. To polarize the sensor, three external electrodes are arranged for three cells, voltages for the three cells for polarization are applied, and electrodes for connecting the external electrodes are then arranged. In the acceleration sensor of preferred embodiments of the present invention, three cells in each of the at least two piezoelectric layers are polarized in the same direction. There is no need for inverting the polarization direction in the longitudinal direction. In the production of the sensor, the electrodes are manufactured in their final shape. The polarization process and the electrode formation process are thus simplified. In addition, manufacturing costs are reduced.
The piezoelectric element of the present invention is not limited to a two-layer structure. The piezoelectric element may include two or more layers in the structure thereof.
The polarization direction and the electrode structure may have various particular conditions. Preferably, all cells of the two external piezoelectric layers are polarized in the same direction. Each of the electrodes on the top and bottom major surfaces of the piezoelectric element includes two longitudinally aligned portions separated at one of the two borders. The electrode between the piezoelectric layers of the piezoelectric element includes two longitudinally aligned portions separated at the other of the two borders. At least one of the electrodes on the top and bottom major surfaces is lead out to one of the longitudinal ends of the piezoelectric element, and the electrode between the piezoelectric layers is lead out to the other of the longitudinal ends of the piezoelectric element.
In this case, the three cells in each of the two layers are serially connected, and then, two layers are connected in parallel. This arrangement results in a voltage sensitivity which is about 1.5 times that of the conventional sensor shown in FIG. 8A, and about 3 times that of the conventional sensor shown in FIG. 9A. Although this arrangement has a charge sensitivity which is about 0.6 times that of the conventional sensor shown in FIG. 8A, and about 0.3 times that of the conventional sensor shown in FIG. 9A, the energy remains substantially unchanged, namely, about 0.9 times that of the conventional sensor. The voltage sensitivity is increased without significantly reducing the energy. An acceleration sensor that is appropriate for use in applications where internal noise level is high is thus provided. Since the polarization directions of all cells are the same, the manufacturing process of the sensor is simplified, and the manufacturing costs are reduced.
The polarization direction and the electrode structure may have other particular conditions. Preferably, the cell of the one piezoelectric layer and the cell of the other piezoelectric layer are polarized in opposite directions. The electrode on the top major surface of the piezoelectric element preferably includes two longitudinally aligned portions separated at one of the two borders. The electrode on the bottom major surface of the piezoelectric element preferably includes two longitudinally aligned portions separated at the other of the two borders. The interlayer electrode of the piezoelectric element includes three longitudinally aligned regions separated at the two borders. The electrode on the top major surface is lead out to one of the longitudinal ends of the piezoelectric element, and the electrode on the bottom major surface is lead out to the other of the longitudinal ends.
In this arrangement, six cells in the at least two piezoelectric layers are serially connected. This arrangement results in a voltage sensitivity which is about 3 times that of the conventional sensor shown in FIG. 8A, and about 6 times that of the conventional sensor shown in FIG. 9A. Although this arrangement has a charge sensitivity which is about 0.3 times that of the conventional sensor shown in FIG. 8A, and about 0.15 times that of the conventional sensor shown in FIG. 9A, the energy remains substantially unchanged, namely, about 0.9 times that of the conventional sensor. The voltage sensitivity is heightened without significantly reducing the energy. An acceleration sensor that is appropriate for use in applications where internal noise level is high is thus provided.
Since the polarization directions of the three cells in each layer are the same, the manufacturing process of the sensor is simplified, and the manufacturing costs are reduced.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.